Denham's amendment proposed with his fellow Southampton MP Alan Whitehead and Slough MP Fiona MacTaggart had argued that it was necessary to proceed with conceptual and design work to maintain the option of an independent deterrent, but that the major decision to commission should come back to Parliament before contracts were placed to build new submarines and warheads (which is when the bulk of the project costs come in).

The Denham amendment approach is a particularly useful one for those who believe in 'something for something' multilateral disarmament, as it would enable the UK to take part in non-proliferation and disarmament talks in which the decisions on Trident renewal could become part of the negotiation. Making it clear that the Commons would vote again would strengthen Britain's international position in those talks, and is a logical step given Gordon Brown's major speech in March which committed Britain to engage seriously in the non-proliferation treaty review process, which the Obama administration has suggested can make significant multilateral progress.

Each of the major parties should commit to a full-scale debate on Trident renewal in the next Parliament, in the light not just of economic considerations but also progress on multilateral disarmament

This was Blair's rather positive signal to the Commons about such an approach two years ago:

Mr. John Denham (Southampton, Itchen) (Lab): Does my right hon. Friend understand that today’s motion on Trident effectively commits this country to the possession of an independent nuclear weapons system for the best part of the next 45 years without the House being guaranteed any future opportunity to consider whether it remains the best strategy? Does he understand that many of us accept the need to ensure our ability to replace the Trident system, but none the less believe that in a fast-changing world this House should be guaranteed the chance to revisit that decision at an appropriate point in the future?

The Prime Minister: I entirely understand my right hon. Friend’s point. If I may put it like this, it is at what I would describe as the reasonable end of the opposition to what the Government are doing. However, let me try to explain why I think we have still got to take this decision today. It is absolutely right that this Parliament cannot bind the decisions of a future Parliament and it is always open to us to come back and look at these issues. He is right to suggest that when we get to the gateway stage—between 2012 and 2014—when we let the main contracts for design and construction, it will always be open to Parliament to take a decision. However, I believe that the reason why we have to take the decision today is that if we do not start the process now, we will not be in the position in 2012 or 2014 to continue with the nuclear deterrent should we wish to do so.

The real dilemma is that we decided rightly or wrongly—but I think rightly—that we should seek parliamentary approval even for the design and concept stage. When we came to the previous Trident nuclear submarine, it was only at a later stage that parliamentary approval was sought. That was much criticised at the time, so we decided that we should seek parliamentary approval at the very beginning of this process. Of course, it is a statement of fact that the gateway takes place at a later stage and in a later Parliament but if we want to spend at least the more limited sum of money now on the concept and design stage, we have to take a decision now.

The House of Commons Select Committee's very useful and informative report (PDF file) endorsed that view of the timing - see its detailed schedule on page 60.

‘This House recognises the changing and uncertain nature of threats to global and British security in the post cold war world; and therefore resolves to take sufficient steps to maintain the UK’s minimum strategic nuclear deterrent beyond the life of the existing system: notes the recommendations of the Defence Select Committee and resolves that further consideration of the strategic case, costs, and design of the system be undertaken; notes also the conclusion of the Defence Select Committee that the government’s strategy for non-proliferation needs development, but resolves to take further steps towards meeting the Uk’s disarmament responsibilities under article V1 of the non-proliferation treaty; and further resolves that the approval of the House be required before main contracts are laid.’

Denham's amendment was not chosen for a vote, with a different amendment for delay from Jon Trickett MP (now PPS to the current Prime Minister chosen instead). Those proposing the 'middle way' amendment voted with the government against rival amendments, on the basis that not conducting design phase would entail de facto unilateral disarmament. That Parliament should vote again before procurement was also the unaninimous view of the Defence Select Committee.

If the White Paper’s proposals to retain and renew the UK’s strategic nuclear deterrent are endorsed, it is essential that the Government keep Parliament informed of the progress of the submarine, missile and warhead programmes. We expect Parliament to be consulted at each significant stage of the programmes before major procurement decisions are made.

The Committee also praised the government for a more open Parliamentary process than any previous government had undertaken. (The Attlee government secretly decided on an independent deterrent, with no Parliamentary vote or public announcement at all).